Howard wasn't unwilling to work with Vanko because he was a communist; he knew that from the getgo. He stopped working with Vanko because Vanko was only interested in making himself rich. Implicit in the exchange was that Vanko wanted to sell the thing off to whoever was the highest bidder.

( and my guess is, the reason Vanko got sent to the gulag on arriving back in Russia wasn't because he defected, its because he couldn't get the arc reactor to work )